The invention relates to a microphone module with the features described in the preamble to claim 1.
A body vibration pickup microphone is known from DE 43 10 793 A1 for installation in a helmet (motorcycle, fire brigade, etc.), with which a flat basic body can be secured by means of an abrasive contact closure system to the inside of the helmet, and the microphone is arranged in a rubber bellows element located on the opposite side of the basic body, so that, when the helmet is put on, it is pressed against the head, and, on speaking, it picks up the surface-borne sound conducted to the surface of the skull. The flat basic body also contains a motherboard with the microphone amplifier.
Also known from DE 195 45 760 C1 is a digital audio device, in which the microphone amplifier is surrounded inside the audio device housing by its own shielding arrangement, in which provision is also made for an A/D converter, so that the complete analog input part of the audio device is screened against electromagnetic scatter interference.
For different application purposes, such as hands-free communications systems in car telephones, mobile radio systems, paging systems, etc., microphone modules are used in which the microphone capsule is assembled directly with a pre-amplifier, so that the weak microphone signals are impeded as little as possible by radiation scatter interference, which impair the understandability of the system. The pre-amplifier is formed on a printed circuit board, on which the microphone capsule is secured by means of a rubber retaining element, and to shield the amplifier a shielding cover is placed onto the circuit board on both sides, which both shields the components on the one side, as well as shielding the printed circuit paths on the other, against environmental interference.
The objective on which the invention is based is of improving such a microphone module in respect of its shielding capacity against interference scatter (immission of electromagnetic waves), with the possibility of retaining the directional characteristics of the microphone, and if possible of improving them.
This objective is achieved by the features described in claim 1. Further embodiments of the invention are characterised in the subclaims.
By the use of printed circuit board lamination coating as shield, instead of for forming the conductor paths, an individual shielding of the lamination coating side of the printed circuit board becomes superfluous, so that only the component fitting side, on which the connection conductors are provided for, needs to be shielded. The invention makes provision in this connection for a half-liner or trough, the walls of which are soldered all around with the circuit board lamination coating, so that a connection is obtained which is absolutely tight against interference scatter. The microphone capsule, half of which is metallised on its rear side, can readily be soldered to this trough or tub, in which situation its metal housing forms a continuous contact with the shield trough, which is intensified by pressure from outside when the module is inserted into a housing. The metallisation on the rear side of the microphone capsule can likewise be easily soldered in with the metal lamination coating of the printed circuit board, as a result of which on the one hand the microphone is secured to the printed circuit board, and, on the other, a good shield effect can also be achieved between the microphone capsule and the printed circuit board.
To achieve a specific directional effect, which is required with microphones of this nature, apertures are formed in the rear wall of the microphone capsule, by means of which the sound pressure on the rear side of the membrane can be influenced in the sense of the desired directional characteristics. If provision is made for corresponding cut-outs at the edge of the printed circuit board, at which the board is connected (soldered) to the microphone capsule, at the points at which the capsule apertures are located, then the directional effect of the microphone capsule will not be impaired, but can even be improved. In order to improve the desired directional effect, in an advantageous embodiment of the invention, the microphone capsule is secured obliquely to the printed circuit board, so that the longitudinal axis of the microphone capsule accordingly forms an angle with the surface of the printed circuit board. An angle of 22xc2x0 has proved to be particularly favourable in this context.